In mass spectrometry, ions are often transported in ion guides within a mass spectrometer (MS) system. The guides are usually devices employing multipole fields to confine the ions along an axis as they pass through the various stages of the instrument.
Without ion guides, many or most of the ions would be lost. If the ions are subjected to collisions with gas molecules for translational cooling and focusing, or for fragmentation, even the multipole ion guides may not be effective in preventing ion losses. The state of the art solution to problems of ion losses and delays in these circumstances is to provide an electric field along the axis of ion motion (axial electric field) in addition to the multipole confinement field.
One class of MS called a tandem mass spectrometer provides an important example. In those instruments (parent) ions are usually selected in a first mass analyzer and then subjected to fragmentation in a collision cell to generate product (daughter) ions. The products are analyzed in a second mass analyzer. When the product ions are formed, they usually have very low kinetic energies, so they are either delayed in passage to the second mass analyzer or many are lost altogether. These problems can be reduced or avoided by applying an electric field directed along the axis of the collision cell to accelerate the slow ions and keep them moving to the exit of the cell and the second mass analyzer.
Methods for producing such axial fields have been described (U.S. Pat. Nos. 4,126,781; 4,283,626; and 5,847,386). However, most means in the art for creating axial fields are somewhat complex, involving multiple layers of resistive materials, insulators and conductors, or complex electrode arrangements that are not easily constructed and aligned. A simpler device with an axial field would be desirable and useful. These and other problems in the art have been addressed by the present invention.